Standards Body 3GPP (The 3rd Generation Partnership Project) advances the standardization of LTE-Advanced (Long Term Evolution Advanced: LTE-A) as an LTE (long Term Evolution) system-compatible next-generation communication standard. In the LTE, a wireless communication base station (E-UTRAN NodeB: eNB) of a network (Evolved Universal Terrestrial Radio Access Network (E-UTRAN) provides one or more communication cells. A wireless communication terminal (User Equipment: UE) belongs to one of the communication cells. Hereinafter, the wireless communication base station (eNB) is called merely “base station”, the communication cell is called merely “cell”, and the wireless communication terminal (UE) is called merely “terminal”.
In the LTE, in order to perform self-optimization of the network (Self Optimizing Network: SON), the terminal provides information at the time of radio link failure (Radio Link Failure: RLF) to the base station. The terminal detects the radio link failure (RLF) in the following three cases.
1. When an instruction of out-of-sync (out-of-sync) is continuously detected N310 times in a physical layer, a count of a given time T310 starts. Then, if in-sync is not continuously detected in the physical layer N311 times, T310 expires, and the terminal detects the RLF. The values of T310, N310, and N311 are notified the terminal of from the base station. If those values are not notified, the terminal uses a predetermined value.
2. When a random access problem (Random Access Problem) is instructed from a media access control (Media Access Control: MAC) layer, the terminal detects the RLF.
3. Arrival at the maximum number of retransmission is instructed from a radio link control (Radio Link Control: RLC) layer, the terminal detects the RLF.
When the terminal maintains an AS security (Access Stratum Security) when detecting the radio link failure (RLF), the terminal starts an RRC connection reestablishment (Radio Resource Control Connection Re-establishment: RRC Connection Re-establishment).
FIG. 12 is a timing chart when the terminal performs the RRC connection reestablishment. Upon detecting the radio link failure (RLF), the terminal detects a suitable cell (Suitable Cell) with the use of cell selecting means, and selects the cell. Then, the terminal transmits an RRC connection reestablishment request message (RRC Connection Reestablishment Request) to the base station of the cell selected by the cell selection. Upon receiving the RRC connection reestablishment request message from the terminal, the base station determines whether to accept the RRC connection reestablishment request message of the terminal, or not. When determining to accept the terminal, the base station transmits an RRC connection reestablishment message (RRC Connection Reestablishment) to the terminal. Upon receiving the RRC connection reestablishment message, the terminal generates an RRC connection reestablishment complete message (RRC Connection Reestablishment Complete). If RLF information related to the RLF that has been finally generated is available, the terminal includes an RLF information available flag (rlf-InfoAvailable) in the RRC connection reestablishment complete message, and sets the flag as true (true). The terminal transmits the RRC connection reestablishment complete message to the base station. With the above operation, the terminal performs the RRC connection reestablishment.
FIG. 13 is a timing chart illustrating an example in which the terminal provides the information at the time of the radio link failure to the base station. The terminal transmits the RRC connection reestablishment complete message in which the RLF information available flag (rlf-InfoAvailable) is included, and the flag is set to the true (true), to the base station. The base station receives the RRC connection reestablishment complete message in which the RLF information available flag is set to the true (true). The base station determines whether the information from the terminal at the time of the radio link failure is necessary, or not. When determining that the information from the terminal is necessary, the base station transmits a terminal information request message (UEInformationRequest) in which an RLF report request flag (rlf-ReportReq) is set to the true (true) to the terminal in order to request transmission of the information at the time of the radio link failure to the terminal. The terminal receives a terminal information request message in which the RLF report request flag Z (rlf-ReportReq) is set to the true (true). The terminal generates an RLF report (rlf-report), generates a terminal information response message (UE Information Response) in which the RLF report (rlf-report) is included, and transmits the terminal information response message to the base station.
FIG. 14 is a diagram illustrating an example of a configuration of the RLF report (rlf-report). As illustrated in FIG. 14, the RLF report (rlf-report) is configured by a measurement result of an own cell finally connected, and a measurement result of a neighbor cell at the time of generating the RLF. The measurement result of the own cell includes a received power (Reference Signal Received Power: RSRP) or a received quality (Reference Signal Received Quality: RSRQ). On the other hand, the measurement result of the neighbor cell is listed for each radio access technology (Radio Access Technology: RAT), and further listed for each frequency. The respective lists are arranged in order from the best cell of the measurement result at the time of generating the RLF. The terminal is set to measure the frequency of the radio access technology which needs to be measured. The measurement result included in the RLF report (rlf-report) is intended for the cell of the frequency of the radio access technology which has been set to be measured (measurement) by the terminal.
The base station or the network can optimize the network on the basis of this information.
In the LTE-A, as a method of an inter-cell interference coordination (Inter-cell Interference Coordination: ICIC), an almost blank subframe (Almost Blank Subframe: ABS) is used for control to prevent an interference between cells in a time domain to protect a resource. In this control, with the provision of a period during which the base station does not transmit the signal, or the transmission power is reduced, to thereby reduce an interference affecting a communication between the other base station and the terminal.
The ABS is characterized by the following elements.
In the ABS a cell specific reference signal (Cell Specific Reference Signal: CRS) is always transmitted.
If the ABS matches a multimedia broadcast multicast service single frequency network (Multimedia Broadcast multicast service Single Frequency Network: MBSFN) subframe, the CRS is not transmitted in a data domain.
If the timing of the ABS matches with a primary synchronization signal (Primary Synchronization Signal: PSS), a secondary synchronization signal (Secondary Synchronization Signal: SSS), a physical broadcast channel (Physical Broadcast Channel: PBCH), a system information block type 1 (System Information Block Type 1: SIB1), paging (Paging), or a positioning reference signal (Positioning Reference Signal: PRS), they are transmitted.
In the ABS, a channel state information reference signal (Channel State Information Reference signal: CSI-RS) can be transmitted.
When the MBSFN subframe is included in the pattern of the ABS, the MBSFN subframe can be used for the time domain inter-cell interference coordination. An example of the method of using the ABS having the above features will be described below.
In FIG. 15, (a) and (b) are diagrams illustrating an example using the ABS. As illustrated in FIG. 15, in addition to a cell arrangement centered on a macrocell in which a coverage area of the cell is wide, a network in which a low power node (Low Power Node: LPN) in which the coverage area of the cell is narrow is arranged within the macrocell has been studied in order to efficiently cover a local area such as an indoor facility a large volume of traffic is concentrated. The low power node is configured by a cell that is managed by a relay node (Relay Node: RN), a picocell that has the same function as that of the macrocell, and is low in the transmission power, or a CSG (Closed Subscriber Group) cell that can be connected with only a specific member without provision of an interface (X2 interfaces) between the base stations.
FIG. 15(a) illustrates an example in which the picocell is arranged within the coverage of the macrocell on the same frequency. For example, the time domain inter-cell interference coordination is used by the terminal that is being connected by the picocell, which is located at an end of the picocell. For example, the time domain inter-cell interference coordination is used to distribute a load of the macrocell into the picocell. In order to protect the specific subframe of the picocell from an interference with the macrocell, the ABS is used by the macrocell. The terminal connected to the picocell performs a radio resource measurement (Radio Resource Measurement: RRM) for a movement control, a radio link monitoring (Radio Link Monitoring: RLM), and a channel state information measurement (Channel State Information measurement: CSI measurement) with the use of the protected resource. The terminal can know the protected resource by allowing a measurement resource restriction (Measurement Resource Restriction) to be notified. As a result, a measurement precision of the picocell under a strong interference with the macrocell can be enhanced by the measurement at timing when the interference with the macrocell is reduced. That is, a range of the picocell can be widened, the connection with the picocell is easily maintained, and the load distribution can be performed.
Also, the terminal that is being connected to the neighbor cell measures the picocell with the use of a measurement resource restriction (Measurement Resource Restriction), thereby being capable of enhancing the measurement precision of the picocell under the strong interference with the macrocell. As a result, handover (handover) of the connection to the picocell is easily performed, and the load distribution can be performed.
FIG. 15(b) illustrates an example in which the CSG cell is arranged within the coverage of the macrocell. For example, the time domain inter-cell interference coordination is used by the terminal that is being connected to the macrocell located on an end of the CSG cell. For example, the terminal that is not allowed to be connected to the CSG cell is used for maintaining the connection with the macrocell.
The CSG cell uses the ABS to reduce the interference, thereby being capable of protecting the subframe of the specific macrocell from the interference. The terminal that is not allowed to be connected to the CSG cell is notified of the protected resource as the measurement resource restriction, and can continue to be connected to the macrocell under the strong interference with the CSG cell by the measurement at the timing when the interference with the CSG cell is reduced. The measurement contents include RRM, RLM, and CSI.